Method and device for removing particles from exhaust gases from internal combustion engines

ABSTRACT

The invention concerns a method and device for removing particles, in particular soot particles, from exhaust gases produced by internal-combustion engines. The invention calls for the particles to be electrostatically charged by an electrode (4). The particles or particle agglomerates are trapped by a fine-mesh metal filter (5). The conducting material (5a), preferably a sintered ferritic metal, of which the filter (5) is made can also be used as a collecting electrode. A high voltage (6) is applied (7, 10) to the electrodes. At intervals, preferably at regular intervals, an electric current is passed (11, 12) through the filter (5), causing the filter material (5a) to heat up to a temperature above the ignition temperature of the particles. The preferred shape of the filter surface is a cylinder, truncated cone and/or cone.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method and a device for the removal ofparticles, in particular particles of soot, from the exhaust of internalcombustion engines, whereby the particles are electrostatically chargedby means of at least one electrode, some of the particles formagglomerates, and the particles and/or particle agglomerates areseparated by means of one or more filters, as well as a device for theremoval of particles, in particular soot particles, from the exhaust ofinternal combustion engines, whereby the device comprises a housing witha feed line and a discharge line for the exhaust, at least one electrodewhich is connected to a high voltage source for the electrostaticcharging of the particles, and a filter body.

2. Description of the Prior Art

The reduction of particulate emissions in exhaust gases plays a majorrole in efforts to further reduce pollution caused by exhaust gases frominternal combustion engines, in particular from Diesel engines. Numerousattempts have therefore been made to remove the particulate emissions,in particular particles of soot, which are contained in exhaust gases.

A survey of the prior art has identified the publication"Rulβfiltertechnik fur Stadtlinienbusse", Internationales Verkehrswesen["Soot Filter Technology for Municipal Buses", InternationalTransportation], 40 (1988), Volume 1, January/February. Ceramic monolithfilters, filter candles with a filter means made of ceramic or wire meshor filters wound with ceramic yarn or a combination of an electriccoagulator and a downstream cyclone are used to remove particulates.

In the last-named system for the removal of particles, an agglomerationof the particles takes place in the electric coagulator, so that theparticles can then be removed in the downstream cyclone, takingadvantage of the centrifugal force on the particles in the receivingchamber of the cyclone, while the gas current is emitted through thesubmerged tube of the cyclone as clean gas into the exhaust. In thisparticle-removal system, however, it is necessary to make separatearrangements for the disposal of the particles removed.

When filters are used, a regeneration or replacement of the filter isnecessary, since the particles which have been removed clog the filter,and the pressure loss caused by the filter increases as the filtercharge increases. The regeneration is performed by burning away theparticles at temperatures which are above the ignition temperature ofthe soot. Since this temperature is not reached in exhaust gases frominternal combustion engines, or is reached only during full loadoperation, either the exhaust gas temperatures must be increased bymotor-driven measures, or the ignition temperature of the exhaust mustbe reduced by additives added independently to the fuel, to the intakeair or to the exhaust, or by catalytic coatings on the filter. Anadditional possibility is a forced regeneration by the application ofheat, e.g. by means of burners.

Metal filters, such as filters made of wire mesh, have the disadvantageof a reduced high-temperature strength, which is insufficient for theremoval of particulates from the above-mentioned exhaust gases. Inparticular, filters made of such material are unsuitable for a thermalregeneration.

U.S. Pat. No. 4,406,119 discloses a method for the removal of particlesfrom the exhaust flow of internal combustion engines, whereby theparticles are negatively charged by means of an electrode with a highvoltage of 10-20 kV, some of the particles form agglomerates and areseparated by means of a filter and a downstream cyclone or heatingelement. The filters consist of porous ceramic or metal meshes ortissues.

EP-A-0 425 433 discloses a process in which charged soot particles arefiltered out in at least one filter with opposite polarity. The filtermaterial consists of an open-pore network structure made of fibers or abase structure which has high temperature resistance, whereby thenetwork structure is designed in the form of foam, porous sinter, fibertissues, braided fibers, non-woven tissue or loose bundles of fibers,and consists of electrically conducting fibers, fibers provided with anelectrically conducting coating, or of such a basic structure.Electrically conducting fibers or basic structures of the filtermaterials can consist of metals, e.g. stainless steel.

JP-A-59 145 314 discloses an apparatus with a high voltage electrode forthe generation of an electrostatic field and a collector made of foamedsteel as a counter electrode to trap the electrically charged particles.

EP-A-0 244 061 discloses a method for the regeneration of a filterloaded with combustible particles of a Diesel engine exhaust, whereby anelectrical current is conducted through the loaded filter, and thefilter is thereby heated above the ignition temperature of theparticles. Several filters are operated-simultaneously in the exhaustflow. During the regeneration, the exhaust gas does not flow through thefilters.

The object of the invention is therefore to create a method and a deviceof the type indicated above which make it possible in a simple manner toeffectively remove particulates from the exhaust gases of internalcombustion engines, in particular of land vehicles or water craftoperated with internal combustion engines, without thereby adverselyaffecting the mobility of the vehicles. During the removal of theparticles, it should be possible to regenerate the filters during theoperation of the internal combustion engine.

SUMMARY OF THE INVENTION

The invention teaches that this object can be achieved by a method inwhich the exhaust gas flows through a sintered metal fine-mesh filterwith at least one precipitation electrode even during the regeneration,and that for the regeneration, an electrical current is conductedthrough the loaded fine-mesh filter, and the fine-mesh filter is therebyheated above the ignition temperature of the particles.

The method according to the invention takes advantage of the hightrapping capacity of fine-mesh filters. The agglomeration alsosignificantly facilitates the separation process. The method can therebybe used over the entire range of particles present in the exhaust gas,such as soot from Diesel engines, but also to remove droplets. Theabove-mentioned filter system is thereby not installed in a bypass flow,but in the main flow of the exhaust gas. Theoretically, severalfine-mesh filters, with different degrees of separation, can beinstalled in cascade fashion. But preferably only one fine-mesh filteris used. A counter electrode as the precipitation electrode is locatedin the fine-mesh filter or fine-mesh filters. In addition to the purelymechanical separation in the filter, there is also an electrostaticseparation of the charged particles.

If the temperature of the exhaust gas in the vicinity of the fine-meshfilter is higher than the ignition temperature of the particles, theparticles are burned off, and there is thus a regeneration of thefine-mesh filter. The invention teaches that an electrical current isperiodically conducted through the fine-mesh filter for the regenerationof the loaded fine-mesh filter. Employing the principle of resistanceheating, the filter material thereby becomes hot and incandescent. Theresult is a regulated heating of the fine-mesh filter above the ignitiontemperature of the particles (approximately between 450 and 600 degreesC.), which results in the burning of the particles and the regenerationof the fine-mesh filter.

The filter material of the fine-mesh filter itself can be advantageouslyused as the precipitation electrode. Thus there is no need for theinstallation of a counter electrode in the fine-mesh filter. Thetrapping capacity of the filter can be increased further.

A ferritic sintered metal is preferably used as the filter material.This material has an extraordinarily high static and dynamic mechanicalstability. It can be used up to very high temperatures, e.g. up to 900degrees C. But the ferritic sintered metals are also characterized byhigh strength in the presence of temperature changes. The ferriticsintered metals used are also chemically stable, e.g. againstalternating oxidizing and reducing atmospheres, such as those which arelikely to be encountered during operation of the filter and during theregeneration process.

The method claimed by the invention preferably has a high voltagebetween 2 and 15 kV applied between the electrodes or between theelectrode and the filter material.

The preferred electrical current is less than 300 A, with an electricalpower between 20 and 140 W, and is conducted through the fine-meshfilter for a period of less than 20 seconds. By observing the valuesindicated above, it is possible to conduct a controlled regeneration onvehicles which have an on-board power supply. Moreover, the removal ofthe particulates is guaranteed during the regeneration process. Thecontrolled regeneration of the invention results in extraordinaryadvantages, with a flexibility and independence specifically forvehicles with internal combustion engines, because the regeneration timecan be selected as a function of the degree of loading of the fine-meshfilter, and since in the method of the invention, no additionaladditives are required to reduce the ignition temperatures, the periodsduring which the filter is out of operation for regeneration can becompletely eliminated. The electrical current can be suppliedperiodically with particular advantage. It is possible, however, to alsosupply the current as a function of the degree of loading of thefine-mesh filter which, for example, can be controlled by means of ameasurement of the drop in the pressure of the exhaust gas.

The object of the invention can be accomplished by means of a device asdisclosed, in which the filter body acting as the counter electrode isintegrated in a sealed manner into the housing, has at least onesintered metal filter surface, and the filter surface is connected bymeans of electrical lines to a current source.

The exhaust gas flow introduced into the housing via the input line isfirst transported past at least one electrode, within causes anelectrostatic charging of the particles in the exhaust gas flow. Theshape of the electrode can be selected to meet the requirements of theapplication in question. Preference is given, however, to the use ofdisc-shaped or ring-shaped electrodes. Several electrodes can be used.There is a counter electrode in the filter body. Preferably, only oneelectrode with a counter electrode is used. The filter body isintegrated in a sealed manner into the housing, so that the entireexhaust flow is forced to flow through the sintered metal filtersurface. The sintered metal filter surface is connected in a conductivemanner to the power source by means of the housing or by means of anelectrical line.

In the device according the invention, the filter surface is connectedto a power source by means of electrical lines. As described above, anelectrical current is conducted intermittently, i.e. discontinuously,through the filter surface, and on account of the specific electricalresistance of the sintered metal filter material used, the filtersurface glows incandescent and heats the soot particles above theignition temperature, thereby burning off the particles.

Advantageously filter surface can consist of ferritic sintered metal.The filter surface is preferably constructed of a nonwoven filterfabric. The geometric shape of the filter surface can correspond tofilter shapes of the prior art. The filter surface is preferably in theshape of a cylinder, a truncated cone or a cone, or represents acombination of one or more filter surfaces in the shape of a cylinder,truncated cone or cone.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in greater detail below, with reference totwo embodiments illustrated in the accompanying figures, wherein:

FIG. 1 is a cross-sectional view of a device according to by theinvention with a cylindrical filter surface; and

FIG. 2 is a cross-sectional view of a device according to by theinvention with a filter surface composed of parts in the shape of acylinder and a truncated cone.

Items which are identical or equivalent in FIGS. 1 and 2 are identifiedby the same reference numbers. In the accompanying figures, thedirection of the exhaust gas flow is indicated by arrows.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In the device illustrated in FIG. 1, the exhaust gas containing theparticles to be removed is conducted via line 2 into the housing 1 of aparticle separator according to the invention. Inside the housing, theexhaust gas is transported past the disc-shaped emission electrode 4.The electrode 4 is connected to a high voltage source 6 by means ofelectrical line 7. The high-voltage line 7 is electrically insulatedfrom the housing by means of an insulator 9. The exhaust gas flow withthe charged particles, or the particle agglomerates which are formed,travels from the electrode 4 to a filter body 5, which includes acylindrical filter surface 5a and a filter holder 5b. The particleseparation takes place on the filter surface 5a, so that a particle-freeexhaust gas leaves the particle separator via line 3.

The particle separation on the filter surface 5a takes place bothmechanically and electrostatically. The filter surface 5a, which is madeof ferritic sintered metal and is itself electrically conducting,thereby acts as a precipitation electrode. There is an electricallyconducting connection to the high voltage source 6 via the filter holder5b, which is also electrically conducting, to the housing 1, and fromthere via the electrical line 10 to the voltage source 6. Theelectrically conducting connection to the power source required forregeneration is guaranteed by means of the electrical lines 11 and 12,whereby line 12 passes through the housing 1 and is electricallyinsulated from the housing 1 by insulator 13.

In contrast to FIG. 1, FIG. 2 does not show a cylindrical filter surface5a, but a filter surface 5a which is composed of parts in the shape of acylinder and a truncated cone. Also in FIG. 2, the electrical connectingline 7 from the electrode 4 to the high voltage source 6 is first guidedcentrally through the filter body 5 and then through the housing 1. Thisspecial routing of the high voltage line 7 prevents a deposition ofparticles on the line 7, and any resulting flashover between theelectrode 4 and the housing 1, to which a different electrical potentialis applied. The necessary electrical insulation with respect to thefilter body 5 and the housing 1 is accomplished by means of insulators 8and 9. Furthermore, in contrast to FIG. 1, in the particle separatorillustrated in FIG. 2, the housing is beveled toward input line 2 andoutput line 3, thereby resulting in an improved flow through theseparator.

Having described certain embodiments of the invention, it is to beunderstood that this may be otherwise embodied within the scope of theappended claims.

We claim:
 1. A method for the removal of particles from the exhaustgases produced by internal combustion engines by electrostaticallycharging the particles by at least one electrode, causing some of theparticles to form agglomerates and trapping the particles and particleagglomerates by at least one filter, said method comprising the stepsof:a. continuously passing the exhaust gases through a sintered metalfine-mesh filter having at least one precipitation electrode even duringregeneration of the filter; and b. conducting an electrical currentperiodically through the sintered metal fine-mesh filter to regeneratethe filter and heat the filter above the ignition temperature of theparticles.
 2. The method as claimed in claim 1 wherein the filtermaterial of the fine-mesh filter itself acts as the precipitationelectrode.
 3. The method as claimed in claim 1 wherein the fine-meshfilter is formed from a ferritic sintered metal.
 4. The method asclaimed in claim 2 wherein the fine-mesh filter is formed from aferritic sintered material.
 5. The method as claimed in claim 1 furtherincluding the step of applying a high voltage between the electrodes. 6.The method as claimed in claim 1 wherein said high voltage is betweenabout 2 and 15 kV.
 7. The method as claimed in claim 2, furtherincluding the step of applying a high voltage between the electrodes. 8.The method as claimed in claim 7 wherein said high voltage is betweenabout 2 and 15 kV.
 9. The method as claimed in claim 4, furtherincluding the step of applying a high voltage between the electrodes.10. The method as claimed in claim 1 wherein an electrical current ofless than 300 A with an electrical power between about 200 and 140 Wpasses through the fine-mesh filter for a period of less than about 20seconds.
 11. The method as claimed in claim 10 wherein the electricalcurrent passes through the filter periodically.
 12. A device for theremoval of particles from the exhaust from internal combustion engines,comprising:a housing with an input line and a discharge line for theexhaust; at least one electrode connected to a voltage source and afilter body; said filter body being integrated in a sealed manner intothe housing, adapted to act as a counter electrode, and having at leastone sintered metal surface; the metal surface being connected by meansof electrical lines to a current source.
 13. The device as claimed inclaim 12 wherein the sintered metal filter surface is connected in aconductive manner to the voltage source.
 14. The device as claimed inclaim 12 wherein the filter surface is formed of ferritic sinteredmetal.
 15. The device as claimed in claim 13 wherein the filter surfaceis formed of ferritic sintered metal.
 16. The device as claimed in claim12 wherein the filter surface is formed of nonwoven filter fabric. 17.The device as claimed in claim 13 wherein the filter surface is formedof nonwoven filter fabric.
 18. The device as claimed in claim 12 whereinthe filter surface is designed in the shape of a cylinder.
 19. Thedevice as claimed in claim 12 wherein the filter surface is designed inthe shape of a truncated cone.
 20. The device as claimed in claim 12wherein the filter surface is designed in the shape of a cone.